Apparatus for positioning optical elements within an optical beam axis are necessary in many applications, in particular in applications concerned with laser manipulation processes such as laser scanning, laser engraving, laser marking, laser ablation or laser etching, but also other application which necessitate deflection of a light beam, whether incoherent, coherent or partially coherent, according in a controllable manner.
EP 0 579 471 A1 discloses a scanning apparatus which includes a scanning mirror having a reflective surface and an electromagnetic apparatus for deflecting the scanning mirror. A coil assembly is mounted onto the scanning mirror and a permanent magnet assembly. A capacitive sensor senses the deflection of the scanning mirror. The capacitive sensor includes a detector for sensing the change in capacitance between a conductive element fixed to the mirror and at least part of the base.
WO 01/78096 A2 discloses an actuator which comprises a planar substrate with two conductors at different potentials. A coil made as a conical helix or two interleaved conical spirals and a magnet generates repelling magnetic fluxes and a second coil creates a third flux causing the ends of the other two coils to move relative to the substrate.
U.S. Pat. No. 4,157,861 A discloses a system which comprises a reflective surface mounted on a baseplate. The baseplate is electromagnetically driven by applied signals to control its angular disposition to a degree of accuracy within fractions of a microradian. Coil springs are attached in pairs to the baseplate to define two orthogonally related axes of movement intersecting at the geometric centre of the baseplate. A pivotal support, preferably a jewel bearing, is positioned at the geometric centre of the baseplate. First and second pairs of permanent magnets extend from the baseplate at opposite equidistant points from the pivotal support to define first and second axes of movement which are also orthogonally related. Associated pairs of electrically conductive coils are disposed around the permanent magnets and spaced from them to permit relative movement.
US 2001/0000130 A1 discloses an oscillation drive unit which consists of an actuator to add power in certain direction for making an antenna to oscillate at a distance from the support portion of an elastic support mechanism. The mechanism supports the antenna at a central point and is attached in a frame so that oscillation can be made biaxially.
US 2003/0058550 A1 discloses a lens holder having a lens, disposed on an upper portion of a suspended yoke plate. Coil printed circuit boards (PCBs) having coils patterned for focusing, tracking and radial tilt driving of the lens holder, are attached on front and rear portions of the lens holder. Magnets positioned at predetermined intervals from the coil PCBs, drive the lens by interaction with the coil PCBs.
WO 2009/106094 A1 discloses a device for positioning an optical element in 1, 2 or 3 dimensions comprising elevation and two-dimensional tilting. Said device comprises a positionable plate whereon said optical element is or can be mounted. Said positionable plate comprises a number of electrical conductive coils serving as actuation elements positioned around the geometrical centre of said positionable plate. A base plate comprising permanent magnets forming electromotive pairs with said coils is supporting said positionable plate by means of a bearing system. When current is driven through said coils over electrical conductive mechanically flexible connections, electromotive forces, all substantially normal to said positionable plate, are formed at said coil positions. Said forces can be compiled in a two-dimensional tilt torque and an elevation force capable, by means of said bearing system, to tilt in two dimensions and/or elevate the positionable plate relative to the base plate. Regulators, comprising error signals derived from a deviation between instantaneous and desired position of said positionable plate relative to said base plate, are used to control said currents.
An apparatus for positioning an optical element typically comprises a positionable part which is mounted to a static base part in a moveable manner by a suspension system, whereby the position of the positionable part with respect to the base part can be changed by means of an actuation system.
These and other prior art devices and methods give rise to some problem, including, but not limited to, the following.
The speed of positioning and repositioning of the optical element could be severely limited in the prior art due to the weight of the positionable part of prior art devices, which can be a consequence of the actuation system of the apparatus, which may necessitate the presence of a heavy iron core on the positionable part for e.g. increasing inductive reactance; furthermore, inductive actuation may lead to considerable energy losses, especially at high positioning speeds, which could necessitate cooling systems.
Another problem with prior art devices and methods is the size of the apparatus and in particular its actuation or suspension system, which can be too large for certain applications or to be implemented into e.g. a table-top or small-scale set-up.
A further problem of prior art devices and methods is that the positionable part is usually moveable around a single axis, or, the positionable part consists of a number of sub-devices, mostly two, each of which are moveable around a single axis and which, when combined, lead to a positionable part which can be rotated around two independent axes. Such a setup is typically used for mirroring optical elements which are thereby capable of deflecting an optical beam to any desired direction within a maximal range. However, such a setup typically requires two or more independent actuation systems, typically one actuation system for each sub-device, which increases the size and mass of the sub-devices, which in turn leads to a reduction of speed (due to e.g. increase of the mass and inertial moments of the sub-devices), a reduction of deflection range or a reduction of maximal beam width (due to e.g. connectivity or wiring difficulties, or e.g. in case of a mirroring optical element which consists of two mirrors, rotatable around perpendicular axis, where the first mirror in the optical path can be kept small, but the second mirror needs to be larger, depending on the maximal deflection angle of the first mirror).
Yet a further issue in prior art devices and methods is the lack of a possible shifting movement of the positionable part. Such a shifting movement refers to a change in position of the positionable part, generally along the optical beam axis. A shifting movement can be used in case a change of optical beam length is required or can be useful. Such can be the case in optical systems comprising e.g. a lens, lensing system, diffraction system, interference system or 3D printing/engravement system.
Another problem arising in prior art devices and methods relates to controlling the movement and/or measuring the position of the positionable part. In order to have optimal control over the movement, a feedback control mechanism based on a measurement of the position of the positionable part can be implemented. Hereby, the position of the positionable part can be made to follow a, preferably pre-determined, target position by measuring the actual position of the positionable part and actuating the movement of the positionable part to minimize the difference between actual position and target position (e.g. by a PID controlling method). Such a method, however, necessitates the presence of a position measurement or sensing system, which is preferably as fast and accurate as possible in order to allow large operational speeds, while keeping costs and safety risks as low as possible.
The present invention aims to resolve at least some of the problems mentioned above.
The invention thereto aims to provide an apparatus for positioning an optical element which can be kept small in size, which comprises an efficient actuation system requiring a very small amount of energy for its operation and hence does not necessitate cooling, which allows movement of the optical element around one or more axes and/or along a longitudinal direction, whereby the position and movement of the positionable part and of the optical element mounted thereon can be controlled fast, safe, cheap and accurately.